Resonators are used in a variety of applications in mobile communications devices. In particular, resonators are often used in filters for mobile communications devices. Resonators for filters generally demand a high quality factor (Q) and selectivity, however, interference in the operation of a resonator due to externalities such as eddy currents often results in sub-optimal performance thereof. FIGS. 1A and 1B show a conventional resonator 10. In particular, FIG. 1A shows an exploded view of the conventional resonator 10 to illustrate one or more details therein, while FIG. 1B shows a cross-sectional view of the conventional resonator 10. The conventional resonator 10 includes a laminate 12, an inductive element 14 on the laminate 12, and a semiconductor die 16 over at least a portion of the inductive element 14 and coupled to the laminate 12. The semiconductor die 16 is a silicon-on-insulator (SOI) semiconductor die including a substrate 18, an insulating layer 20 on the substrate 18, and a device layer 22 on the insulating layer 20 opposite the substrate 18. While not shown, one or more semiconductor devices (e.g., transistors, capacitors, etc.) are located in the device layer 22 and connect to the inductive element 14 (and possibly other components on the laminate 12 or on semiconductor dies attached to the laminate 12), for example, via one or more flip-chip pillars 24, which may connect to one or more bond pads 26 on the laminate 12 via a soldering process. In particular, one or more capacitive elements (not shown), which may be metal-insulator-semiconductor (MIS) capacitors located in the semiconductor die 16, may connect to the inductive element 14 directly or via one or more switching elements (e.g., transistors) to form the conventional resonator 10.
In operation, a magnetic field generated by the inductive element 14 extends outwards towards the semiconductor die 16. Because the conventional resonator 10 is generally used for radio frequency (RF) applications, the magnetic field is time-varying. Accordingly, the magnetic field induces a circular current, known as an eddy current, in the various layers of the semiconductor die 16. Generally, the eddy current induced by a magnetic field is proportional to the strength of the magnetic field, the area available to propagate the eddy current, and the rate of change of the magnetic field, and is inversely proportional to the resistivity of the material in which the eddy current is induced. Since the substrate 18, the insulating layer 20, and the device layer 22 generally have a relatively high resistivity, eddy currents due to the inductive element 14 would typically not be problematic. However, a common phenomenon in SOI semiconductor die is the accumulation of charge carriers at the interface between the substrate 18 and the insulating layer 20. This generates a large cross-sectional area with a relatively low resistivity, resulting in a strong eddy current therein. The eddy current induced in the semiconductor die 16 in turn induces an opposing magnetic field to that provided by the inductive element 14, which interferes with the magnetic field of the inductive element 14 and thereby reduces the quality factor of the conventional resonator 10.
One way to reduce the impact of eddy currents on the performance of the inductive element 14 is by using an electromagnetic shield to limit the penetration of the magnetic field from the inductive element 14 into the semiconductor die 16. Accordingly, FIG. 2 shows a cross-sectional view of the conventional resonator 10 further including a shield 28 between the inductive element 14 and the semiconductor die 16. The shield 28 is on the device layer 22, and includes a number of openings 30 to allow the flip-chip pillars 24 to pass through. The shield 28 includes a conductive base layer 32 and an anti-reflective coating 34. As will be appreciated by those of ordinary skill in the art, the anti-reflective coating 34 is generally provided on the semiconductor die 16 in order to increase the resolution of photolithography processes that may be performed on the die. That is, the anti-reflective coating 34 is the result of a standard fabrication process of the semiconductor die 16 and thus is present in a vast majority of commercially available semiconductor die. Notably, the conductive base layer 32 has a relatively low resistivity, while the anti-reflective coating 34 has a higher resistivity. As discussed above, the strength of an eddy current induced in a particular material is inversely proportional to the resistivity thereof. However, the effect of an induced eddy current on the magnetic field that caused the current is proportional to both the strength of the eddy current and the resistivity of the material in which it is induced. Accordingly, eddy currents induced in the anti-reflective coating 34 may be especially problematic, and result in significant reductions in the quality factor of the conventional resonator 10.
In light of the above, there is a need for resonator circuitry with improved performance.